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Gaseous Exchange

Objectives

By the end of this lesson you should be able to:

Define the term gaseous exchange

State the importance of gaseous exchange in plants

Explain three mechanisms of opening and closing of stomata

Describe the internal section of the root, stem and leaf of
terrestrial plants.

Describe the internal section of the root, stem. and leaf of
aquatic plants

Compare the internal cross section of the leaf, stem and root of
terrestrial and aquatic plants

Identify the various respiratory structures in animals.

State how the characteristics of respiratory surfaces adapt them
to their function.

Explain the mechanism of gaseous exchange in protozoa, insects,
fish, frogs and mammals.

Explain the factors determining energy requirements in man.

State the causes, symptoms and preventive measures for
respiratory diseases.

GASEOUS EXCHANGE

In this topic you will learn how plants and animals exchange oxygen
and carbon dioxide between them and their environment. You will also
learn about the various structures involved and their adaptations.

Background Information

During introduction to biology topic you
learnt that gaseous exchange is one of the characteristics of living
organisms. You learnt that some plants grow on land while others grow in
water. You also learnt that during photosynthesis plants take in Carbon
IV oxide and give out oxygen gas. Animals take in air rich in oxygen and
give out air rich in carbon IV oxide. In this topic we are going to
learn how plants take in useful gases and remove waste gases from the
body. We will also learn how animals exchange gases in addition to
diseases that affect the gaseous exchange system.

Definition of Gaseous Exchange

Gaseous exchange refers to the diffusion of
respiratory gases across the respiratory surfaces. The respiratory gases
are oxygen and carbon iv oxide gas. The respiratory surfaces vary from
organism to organism. They are the actual sites where gases diffuse into
and out of the body of the organism.

Importance of Gaseous Exchange

Gaseous exchange enables living organisms to
obtain useful gases and remove the waste gases from their bodies. Plants
and animals obtain oxygen which they need for respiration while giving
out carbon IV oxide as a waste gas.Plants utilize the carbon IV oxide
produced from respiration for the process of photosynthesis in presence
of light.

Structure and function of guard cells and stomata

Guard cells are special epidermal cells found on leaves.

The illustration shows guard cells with surrounding epidermal cells

There are two guard cells for every stoma which are placed opposite
each other. Their inner walls are thick while the outer walls are thin.
They contain chloroplasts.

Mechanism of Closing and Opening of
Stomata

There are three theories which have
been put across to explain the mechanism of opening and closing of the
stomata:

Photosynthetic theory,

pH theory

Potassium ion theory.

Photosynthetic theory

You learnt earlier during the topic
nutrition that enzyme controlled reactions are reversible. Enzymes are
protein and therefore sensitive to pH. pH can be alkaline, neutral or
acidic.

We learnt that Carbon IV oxide
dissolves in water to form acidic solutions.In presence of light
photosynthesis occurs in the guard cells due to presence of chloroplast.
Sugars are formed in the guard cells. The guard cells develop a higher
osmotic pressure (tendency to draw in water by osmosis) compared to the
neighboring epidermal cells which do not carry out photosynthesis. The
guard cells draw in water from the epidermal cells by osmosis and swell
becoming turgid.

The thin outer walls of guard cells
stretch more than the thick inner walls. A space is left between the
guard cells as they move apart and at this stage the stomata is said to
be open.

In absence of light

In absence of light photosynthesis
does not occur in the guard cells. Sugars formed in presence of light
are converted into starch by enzymes in the guard cells. The guard cells
lose their osmotic pressure. Water moves out of the guard cells into the
epidermal cells by osmosis, and they become flaccid. The thick inner
walls straighten up as the thin outer walls lose their stretch. The
space between the guard cells becomes small and the stoma closes up.
Note that the stoma does not completely close up. A small aperture is
left to allow gaseous exchange to continue taking place in absence of
light.

pH theory

In presence of light Carbon IV oxide is used up during the process of
photosynthesis. Less carbon IV oxide is found in the guard cells. The pH
of the guard cells rises i.e. it becomes less acidic. This favors the
conversion of starch into sugars by enzymes in the guard cells. Presence
of sugars makes the guard cells to develop an osmotic pressure. The
guard cells gain water by osmosis from the surrounding epidermis cells
to become turgid. The thin outer walls of the guard cells stretch more
than the thick inner walls. The thick inner walls bulge away from each
other and the space between the two guard cells increase. The stoma
opens.

In absence of light photosynthesis does not take place in the guard
cells. Carbon IV oxide released during respiration accumulates in the
guard cells and dissolves forming an acidic solution.

A low pH (acidic) favors the conversion of sugars into starch by
enzymes in the guard cells. The guard cell develops higher osmotic
pressure compared to the surrounding epidermal cells. As a result they
lose water by osmosis and become flaccid. The thick inner walls
straighten up as the thin outer walls lose their stretch. The space
between the guard cells becomes small and the stomata close.

Potassium ion theory

According to this theory the stomata opens when potassium ions are
actively pumped from the neighbouring epidermal cells into the guard
cells. Water moves from the epidermal cells into the guard cells by
osmosis. The guard cells become turgid and the stomata open. The stomata
close when the potassium ions diffuse from the guard cells to the
epidermal cells and as a result the guard cells lose water by osmosis

Gaseous Exchange in Terrestrial Plants

Terrestrial plants can be grouped into two types:

Xerophytes

Mesophytes

Xerophytes

Xerophytes are plants that grow in areas that subjects them to harsh
climatic condition i.e. little water and extreme temperature. They are
described as xerophytic plants e.g. Acacia, Aloe and pine. In this
session we will learn how the structures in Acacia plants adapt it to
carry out gaseous exchange.

Internal structure of the leaf of an arid or semi arid habitat plant

The
leaf has the following features:

Few stomata and of small size

Stomata are only on the lower epidermis

Small intercellular air spaces in the spongy mesophyll region

A small percentage of gaseous exchange occurs in the lenticels

These are small openings on the stem. Cells in the lenticel are thin
walled, and are loosely packed to create air spaces for gaseous
exchange. Oxygen diffuses in while carbon IV oxide diffuses out.

Mesophytes

Mesophytes are plants that thrive in areas that have moderate
climatic conditions.

Leaves of mesophytes are broad. They have many stomata on both upper
and lower epidermis. The stomata are large in size. Their spongy
mesophyll tissues have large air spaces.

Stems of mesophytes

Their stems have numerous lenticels for gaseous exchange as in
xerophytes.

Roots

The root hair cells are thin walled for faster rates of gaseous
exchange. They hae a projection, the root hair, which increases the
surface area or gseous exchange. Their epidermal cells are thin walled
for faster rates of gaseous exchange.

Aquatic Plants

Aquatic plants are adapted to grow in water. They are divided into
two:

Hydrophytes: plants growing in fresh water.

Halophytes: Plants growing in saline water

These plants grow under low oxygen and carbon IV oxide concentration
conditions. Some aquatic plants are submerged, others are emergent and
still others are floating.

In this lesson we are going to learn how the stems, roots and leaves
of aquatic plants are adapted for gaseous exchange.

Cross section shows the following features:

Numerous stomata on upper epidermis only

Large intercellular air spaces in spongy mesophyll region

Presence of aerenchyma tissues

Presence of air bladder

Broad leaves to increase surface area for gaseous exchange.

Thin leaves to reduce the distance over which diffusion takes place

The stomata are large in size

A cross section of a Floating Hydrophyte Stem

Note that the stems have large aerenchyma tissues. Conducting tissues
are at the center of the stem to create space for aerenchyma tissues.
Roots are fibrous to create a large surface area for gaseous exchange

Gaseous Exchange in Emergent
Plants

These are plants that grow in
water logged soils. Their leaves and stems match those of mesophytes.
The root of emergent saline water plants such as mangroves have special
breathing roots called pneumatophores for gaseous exchange.

Gaseous Exchange in Submerged
Plants

These are plants that grow inside water. An example is Elodea. Their
leaves are generally thin to increase the rate of diffusion of
respiratory gases. The leaves are deeply dissected in some species to
increase the diffusion surface area. They have large and numerous
parenchyma tissues which are sites for gaseous exchange. They use the
epidermis for gaseous exchange. These plants however lack guard cells
for gaseous exchange. The submerged plants also tend to lack stems and
roots.

Gaseous exchange in Protozoa

Gaseous
exchange in unicellular organisms such as amoeba is by simple diffusion
across the respiratory surface which is the cell membrane. This is
because of their large surface area too volume ratio. Usually the water
surrounding the unicellular organism has a higher concentration of
oxygen compared to inside of the unicellular organism. The difference in
concentration gradient causes oxygen to diffuse from the water into the
amoeba while carbon IV oxide which is high in concentration in the
amoeba diffuses into the surrounding water.

An illustration of surface area
to volume ratio using a large and a small chalk cube.

A large cube of chalk and a
small cube of chalk are put in colored ink for a few minutes and then
removed. A cross section af both pieces is made. It is observed that in
the small cube ink has spread to all parts but in the large cube of
chalk a section of the cube has no ink.This explains the why diffusion
used for gaseous exchange can be sufficient in small organism as
compared to large ones.

Gaseous Exchange in Insects

You learnt in form one that all
living organisms carry out gaseous exchange. Insects have a well
developed breathing system called the tracheal system. We shall study
the breathing mechanism in insects

In insects air enters the body through small
opening on the thorax and abdomen called spiracles. The spiracles opens
up to a system of branching tubes called the tracheal system. These
tubes are of two types:

Trachea: Are wide and have rings chitin

Tracheole-narrow and lack rings of chitin

These tubes are permeable to gases. They are moist and have one cell
thick (thin) wall. They are fluids filed for gases to dissolve and
diffuse. The spiracles have hair around the opening and valves. The
hairs prevent lose of moisture from the tracheal system and also prevent
entry of dust particles into the tracheal system. The valves control the
entry and exit of the respiratory gases.

Mechanism of Gaseous Exchange in Insects

Gaseous exchange occurs during inhalation and
exhalation

Inhalation

This involves air being drawn in through the
thoracic spiracles. This occurs when the abdominal muscles relax causing
an increase in the abdominal volume and decrease in the pressure. The
valves in the thoracic spiracles open air is drawn in. they then close
and due to muscle movement air is forced along the tracheal system.
Oxygen dissolves in tracheal fluid and diffuses into the tissues due to
diffusion gradient. Carbon IV oxide diffuses from the tissues to the
trachea fluid due to diffuse gradient.

Exhalation

During exhalation air is drawn out through the abdominal spiracles.
This occurs when the abdominal muscles contract causing an increase in
the abdominal pressure and a reduction in the volume. The valves on the
abdomen spiracles open while those of thorax close air moves. Animation
showing abdominal muscles contract and subsequent shortening of the
abdomen.

Gaseous Exchange in FishFish use gills for gaseous exchange

During gaseous exchange the fish opens its
mouth. Muscular contractions bring about the lowering of the floor of
the mouth. The volume of the mouth cavity increases while the pressure
decreases, water flows into the mouth. The operculum on both sides of
the head bulges outwards causing reduction in pressure in the gill
cavity. Water containing dissolved oxygen flows from the mouth cavity to
the gill chamber.

Oxygen diffuses from the water flowing over the gills
into the blood capillaries in the gill filaments This is due to
diffusion gradient. Carbon IV Oxide diffuses from the blood capillaries
to the water in the opercula cavity as a result of diffusion gradient.

After gaseous exchange in the gills the water rich in Carbon IV oxide
is expelled. The fish close its mouth relaxation of the muscles causes
the floor of the mouth to be raised involving the pressure and reduce
the volume forcing any remaining water in the mouth cavity to flow
towards the gill chamber and out of the gill chamber through the free
edge of the operculum.

As water flows out blood in the gill filaments
vessels flow in the opposite direction a process called counter-current
mechanism which increases efficiency of gaseous exchange.

Gaseous Exchange in Amphibians

Amphibians e.g. toads, frogs and newts
use the the skin, the buccal cavity and the lungs for gaseous exchange

Gaseous exchange over the
Skin

Gaseous exchange through the skin is also
called cutaneous gaseous exchange. The skin of a frog is thin and highly
vascularised. It is kept moist at all times for gases to dissolve. The
atmosphere has more oxygen than the blood in the skin of frog. Oxygen
diffuses from the air across the skin and capillary membranes and into
the blood.

Carbon IV oxide is at high concentration in the blood of the
skin of the frog as compared to the atmosphere. It consequently diffuses
out from the blood , through the capillaries and through the skin
membranes into the atmosphere.

Gaseous
exchange over the Buccal Cavity

Gaseous exchange also takes place over the membranes of the buccal
cavity. This is also known as a mouth cavity. The walls are lined with a
thin membrane and kept moist. They also have an extensive network of
blood capillaries.

Ventilation

Inhalation

During Inhalation the mouth is closed
and nostrils are open. The floor of the mouth cavity is lowered causing
an increase in volume and lowering of pressure. This causes the air to
enter through the nostrils into the mouth cavity, oxygen diffuses into
the moist skin membranes and capillary membranes and is transported to
all parts of the body.

Ventilation in buccal cavity

Exhalation

Carbon iv oxide diffuses from blood a cross the membrane of the
capillaries and skin into the air in the mouth cavity .To expel the air
the floor of the mouth cavity is raised causing the pressure to increase
and the volume decreases. Meanwhile the mouth is closed and the nostrils
open, hence air is expelled.

Gaseous exchange over the
Lungs

Frogs have a pair of lungs hanging in the cavity. Air from the buccal
cavity is received into the lungs through a structure called glottis. It
open into a larynx which is connected to a trachea. The trachea branches
into short tubes called bronchi (singular bronchus) which extends into
each lung .The lungs comprises of airspaces referred to as alveoli. They
have a film of moisture and a rich network of capillaries to allow gases
to dissolve and increase the surface area for gaseous exchange.

Ventilation in the lungs.

During inhalation the nostrils open while mouth and glottis are
closed. The floor of the mouth is lowered increasing the volume of the
mouth cavity while the pressure is reduced. Air rich in oxygen enters
through nostrils into the mouth cavity. The nostrils and mouth then
closes. The floor of the mouth cavity is raised forcing open the glottis
and air is forced into the lungs. Oxygen diffuses into the blood across
the membranes of the alveoli and blood capillaries. Oxygen is used up in
respiration and carbon iv oxide is produced and diffuses out of
capillaries into the alveolar space.

Exhalation

Occurs when nostrils are closed and air is sucked from the lungs into
the mouth cavity .

The glottis closes, nostrils open while the floor of the mouth is
raised forcing air out of the mouth cavity into the atmosphere through
the nostrils.

Gaseous exchange in Mammals

Structure Of Breathing System in Man

Breathing system in man consists of the following structures:
nostrils, trachea, lungs, diaphragm and chest cavity made of ribs and
intercostal muscles.

Trachea

The trachea is a tube made up of rings of cartilage which ensures
that it does not collapse during breathing. The lumen of the trachea is
lined with ciliated epithelium. The cilia beat in waves and move the
mucus and foreign particles towards the pharynx away from the lungs. As
the trachea enters the lungs it divides into two branches called
bronchi(singular bronchus)

Lungs

Lungs are found in the chest cavity. They are enclosed in a double
membrane known as the pleural membrane. One part of the membrane adheres
tightly to the lungs and other covers the inside of the thoracic cavity.
The space between those membranes is known as the pleural cavity. It is
filled with pleural fluid which reduces friction making the lungs move
freely in the chest cavity. Within the lungs, each bronchus divide into
small tubes called bronchioles in groups of tiny air sacs called alveoli
(singular alveolus) hence the spongy nature of the lungs. Alveolus is
covered by a fine network of blood capillaries.

The air enters through the nostrils into the trachea and bronchus and
bronchioles. As the air passes through the nostrils the hairs trap dust
particles; air is moistened and warmed making it advantageous to breath
in through the nose.

As air passes through this trachea, bronchus the cilia and mucus
produced by the epithelium cells trap dust and bacteria and carries them
away towards the esophagus. The tracheal branches have rings of
cartilage which keeps them open during ventilation.

Gaseous exchange in alveolus

The alveoli are thin single celled air sacs found at the terminal end
of bronchioles. They are vascularized, moist and numerous in number.
Gaseous exchange at the alveolus takes place by diffusion.

Gaseous Exchange

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